When I try to blender two scene strips using Alpha Over, it looks like white is being blended for the alpha on the top scene. Clicking Premultiply helps, but the empty area of the scenes (the sky color) is still being lightened. How do you combine two scene strips without this happening?
Although I use nodes, not the VSE, to do alpha-comp’ing these days, the principle remains: things that you are compositing should not have any “sky color” at all.
Every thing that you composite into the shot should have nothing outside of itself. The elements of each layer should be there, with a nonzero alpha-value, and every element which is not part of the layer should have an alpha of 0.0.
If you want to have “a sky color,” you put that into the background plate.